Microwave filters of the coaxial type are usually provided with coaxially structured terminals for connection with external circuit modules and such connection is provided by a coaxial cable or the like. Each terminal is provided with a probing electrode which projects into the filter interior for energy transfer. In cases where a system is desired to meet low-loss requirements, a hybrid construction is often employed in which such microwave filters are connected to strip line circuit modules such as integrated circuits. This requires a specially designed connector that provides mode conversion between different field configurations of the dominant mode. However, an abrupt structural change tends to occur at the connecting point, so that impedance mismatch is introduced.